722 research outputs found
The mass ratio distribution of short period double degenerate stars
Short period double degenerates (DDs) are close white dwarf - white dwarf
binary stars which are the result of the evolution of interacting binary stars.
We present the first definitive measurements of the mass ratio for two DDs,
WD0136+768 and WD1204+450, and an improved measurement of the mass ratio for
WD0957-666. We compare the properties of the 6 known DDs with measured mass
ratios to the predictions of various theoretical models. We confirm the result
that standard models for the formation of DDs do not predict sufficient DDs
with mass ratios near 1. We also show that the observed difference in cooling
ages between white dwarfs in DDs is a useful constraint on the initial mass
ratio of the binary. A more careful analysis of the properties of the white
dwarf pair WD1704+481.2 leads us to conclude that the brighter white dwarf is
older than its fainter companion. This is the opposite of the usual case for
DDs and is caused by the more massive white dwarf being smaller and cooling
faster. The mass ratio in the sense (mass of younger star)/(mass of older star)
is then 1.43+-0.06 rather than the value 0.70+-0.03 given previously.Comment: Accepted for publication in MNRA
The triple degenerate star WD1704+481
WD1704+481 is a visual binary in which both components are white dwarfs. We
present spectra of the H-alpha line of both stars which show that one component
(WD1704+481.2 = Sanduleak B = GR 577) is a close binary with two white dwarf
components. Thus, WD1704+481 is the first known triple degenerate star. From
radial velocity measurements of the close binary we find an orbital period of
0.1448d, a mass ratio, q=Mbright/Mfaint of q=0.70+-0.03 and a difference in the
gravitational redshifts of 11.5+-2.3km/s. The masses of the close pair of white
dwarfs predicted by the mass ratio and gravitational redshift difference
combined with theoretical cooling curves are 0.39+-0.05 solar mass and
0.56+-0.07 solar masses. WD1704+481 is therefore also likely to be the first
example of a double degenerate in which the less massive white dwarf is
composed of helium and the other white dwarf is composed of carbon and oxygen.Comment: 5 pages, 4 figure
SDSS J080531.84+481233.0: An Unresolved L Dwarf/T Dwarf Binary
SDSS J080531.84+481233.0 is a peculiar L-type dwarf that exhibits unusually
blue near-infrared and mid-infrared colors and divergent optical (L4) and
near-infrared (L9.5) spectral classifications. These peculiar spectral traits
have been variously attributed to condensate cloud effects or subsolar
metallicity. Here I present an improved near-infrared spectrum of this source
which further demonstrates the presence of weak CH4 absorption at 1.6 micron
but no corresponding band at 2.2 micron. It is shown that these features can be
collectively reproduced by the combined light spectrum of a binary with L4.5
and T5 components, as deduced by spectral template matching. Thus, SDSS
J080531.84+481233.0 appears to be a new low-mass binary straddling the L
dwarf/T dwarf transition, an evolutionary phase for brown dwarfs that remains
poorly understood by current theoretical models. The case of SDSS
J080531.84+481233.0 further illustrates how a select range of L dwarf/T dwarf
binaries could be identified and characterized without the need for high
angular resolution imaging or radial velocity monitoring, potentially
alleviating some of the detection biases and limitations inherent to such
techniques.Comment: 11 pages, 4 figures, accepted by A
Orbital periods of the binary sdB stars PG0940+068 and PG1247+554
We have used the radial velocity variations of two sdB stars previously
reported to be binaries to establish their orbital periods. They are
PG0940+068, (P=8.33d) and PG1247+554 (P=0.599d). The minimum masses of the
unseen companions, assuming a mass of 0.5 solar masses for the sdB stars, are
0.090 +/- 0.003 solar masses for PG1247+554 and 0.63 +/- 0.02 solar masses for
PG0940+068. The nature of the companions is not constrained further by our
data.Comment: 5 pages, 2 figure
Radial velocity measurements of white dwarfs
We present 594 radial velocity measurements for 71 white dwarfs obtained
during our search for binary white dwarfs and not reported elsewhere. We
identify three excellent candidate binaries, which require further observations
to confirm our preliminary estimates for their orbital periods, and one other
good candidate. We investigate whether our data support the existence of a
population of single, low mass (<~0.5 solar masses) white dwarfs (LMWDs). These
stars are difficult to explain in standard models of stellar evolution. We find
that a model with a mixed single/binary population is at least ~20 times more
likely to explain our data than a pure binary population. This result depends
on assumed period distributions for binary LMWDs, assumed companion masses and
several other factors. Therefore, the evidence in favour of the existence of a
population of single LMWDs is not sufficient, in our opinion, to firmly
establish the existence of such a population, but does suggest that extended
observations of LMWDs to obtain a more convincing result would be worthwhile .Comment: 14 pages, 4 Figures. Accepted for publication in MNRAS. Added
Institutio
Resolved Spectroscopy of M Dwarf/L Dwarf Binaries. II. 2MASS J 17072343-0558249AB
We present IRTF SpeX observations of the M/L binary system 2MASS
J17072343-0558249. SpeX imaging resolves the system into a 1"01+/-0.17 visual
binary in which both components have red near infrared colors. Resolved
low-resolution (R~150) 0.8-2.5 micron spectroscopy reveals strong H2O, CO and
FeH bands and alkali lines in the spectra of both components, characteristic of
late-type M and L dwarfs. A comparison to a sample of late-type field dwarf
spectra indicates spectral types M9 and L3. Despite the small proper motion of
the system (0"100+/-0"009 yr^{-1}), imaging observations over 2.5 yr provide
strong evidence that the two components share common proper motion. Physical
association is also likely due to the small spatial volume occupied by the two
components (based on spectrophotometric distances estimates of 15+/-1 pc) as
compared to the relatively low spatial density of low mass field stars. The
projected separation of the system is 15+/-3 AU, similar to other late-type M
and L binaries. Assuming a system age of 0.5-5 Gyr, we estimate the masses of
the binary components to be 0.072-0.083 and 0.064-0.077 M_sun, with an orbital
period of roughly 150-300 yr. While this is nominally too long a baseline for
astrometric mass measurements, the proximity and relatively wide angular
separation of the 2MASS J1707-0558AB pair makes it an ideal system for studying
the M dwarf/L dwarf transition at a fixed age and metallicity
Kinematic structure in the young Sigma Orionis association
We have used precise radial velocity measurements for a large number of
candidate low-mass stars and brown dwarfs, to show that the young Sigma Ori
``cluster'' consists of two spatially superimposed components which are
kinematically separated by 7 km/s in radial velocity, and which have different
mean ages. We examine the relationship of these two kinematic groups to other
populations in the Orion OB1 association and briefly discuss the consequence of
mixed age samples for ongoing investigations of the formation and evolution of
low-mass objects in this much-observed region.Comment: MNRAS Letter in pres
Low Luminosity Companions to White Dwarfs
This paper presents results of a near-infrared imaging survey for low mass
stellar and substellar companions to white dwarfs. A wide field proper motion
survey of 261 white dwarfs was capable of directly detecting companions at
orbital separations between and 5000 AU with masses as low as 0.05
, while a deep near field search of 86 white dwarfs was capable of
directly detecting companions at separations between and 1100 AU with
masses as low as 0.02 . Additionally, all white dwarf targets were
examined for near-infrared excess emission, a technique capable of detecting
companions at arbitrarily close separations down to masses of 0.05 .
No brown dwarf candidates were detected, which implies a brown dwarf
companion fraction of % for white dwarfs. In contrast, the stellar
companion fraction of white dwarfs as measured by this survey is 22%,
uncorrected for bias. Moreover, most of the known and suspected stellar
companions to white dwarfs are low mass stars whose masses are only slightly
greater than the masses of brown dwarfs. Twenty previously unknown stellar
companions were detected, five of which are confirmed or likely white dwarfs
themselves, while fifteen are confirmed or likely low mass stars.
Similar to the distribution of cool field dwarfs as a function of spectral
type, the number of cool unevolved dwarf companions peaks at mid-M type. Based
on the present work, relative to this peak, field L dwarfs appear to be roughly
2-3 times more abundant than companion L dwarfs. Additionally, there is no
evidence that the initial companion masses have been altered by post main
sequence binary interactions.Comment: 149 pages, 59 figures, 11 tables, accepted to ApJ Supplement
PG 1018â047 : the longest period subdwarf B binary
About 50 per cent of all known hot subdwarf B stars (sdBs) reside in close (short-period) binaries, for which common-envelope ejection is the most likely formation mechanism. However, Han et al. predict that the majority of sdBs should form through stable mass transfer leading to long-period binaries. Determining orbital periods for these systems is challenging and while the orbital periods of âŒ100 short-period systems have been measured, there are no periods measured above 30 d. As part of a large programme to characterize the orbital periods of sdB binaries and their formation history, we have found that PG 1018â047 has an orbital period of 759.8 ± 5.8 d, easily making it the longest period ever detected for a sdB binary. Exploiting the Balmer lines of the subdwarf primary and the narrow absorption lines of the companion present in the spectra, we derive the radial velocity amplitudes of both stars, and estimate the mass ratio MMS/MsdB= 1.6 ± 0.2. From the combination of visual and infrared photometry, the spectral type of the companion star is determined to be mid-K
Binaries and the L Dwarf/T Dwarf Transition
High-resolution imaging has revealed an unusually high binary fraction
amongst objects spanning the transition between the L dwarf and T dwarf
spectral classes. In an attempt to reproduce and unravel the origins of this
apparent binary excess, I present a series of Monte Carlo mass function and
multiplicity simulations of field brown dwarfs in the vicinity of the Sun.
These simulations are based on the solar metallicity brown dwarf evolutionary
models and incorporate empirical luminosity and absolute magnitude scales,
measured multiplicity statistics and observed spectral templates in the
construction and classification of composite binary spectra. In addition to
providing predictions on the number and surface density distributions of L and
T dwarfs for volume-limited and magnitude-limited samples, these simulations
successfully reproduce the observed binary fraction distribution assuming an
intrinsic (resolved) binary fraction of 11(+6)(-3)% (95% confidence interval),
consistent with prior determinations. However, the true binary fraction may be
as high as 40% if, as suggested by Liu et al., a significant fraction of L/T
transition objects (~66%) are tightly-bound, unresolved multiples. The
simulations presented here demonstrate that the binary excess amongst L/T
transition objects arises primarily from the flattening of the luminosity scale
over these spectral types and is not inherently the result of selection effects
incurred in current magnitude-limited imaging samples. Indeed, the existence of
a binary excess can be seen as further evidence that brown dwarfs traverse the
L/T transition rapidly, possibly driven by a nonequilibrium submergence of
photospheric condensates.Comment: 51 pages, 14 figures; accepted for publication in Ap
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